Parametric processes in highly nonlinear fiber (HNLF) are used in parametric amplification, wavelength conversion, ultrafast optical sampling and multiple channel penalty mitigation. A unique characteristic of two-pump parametric interaction in HNLF enables multispectral switching, which is not attainable by conventional one-pump devices. While significant progress in advancing the art of parametric design has been made to date, a number of practical issues still need to be resolved. The occurrence of Brillouin scattering represents a chief obstacle towards higher parametric performance since it limits the usable pump powers, and consequently, the available parametric gain/efficiency. In parametric fiber applications requiring high continuous wave (cw) optical power, SBS (stimulated Brillouin scattering) must be suppressed. This is commonly accomplished by frequency modulation, digital phase modulation, or analog phase modulation of the cw light to suppress the carrier component and broaden the pump spectrum. Frequency modulation of the pump light is typically accomplished by repetitively dithering the frequency of the source laser. Digital phase modulation is accomplished by means of an external (separate device) phase modulator driven repetitively by an electronic signal such as a PRBS (pseudo-random bit stream), typically at fairly high data rates (>1 Gb/s). Analog phase modulation is accomplished by means of an external phase modulator driven repetitively by an electronic signal composed of multiple harmonics. While applications requiring high cw optical power use pump spectral broadening in order to increase the Brillouin threshold, in one-pump parametric architectures this causes excessive idler spectral broadening, which cause a fundamental impairment. This basic limitation can be, in principle, completely removed in the two-pump architecture by counter phasing the modulation of the pumps, when using PRBS or multiple-harmonic pump phase modulation or frequency modulation. In such an arrangement, when the phase of one pump is increased the phase of the other pump is decreased and vice versa, thereby maintaining a constant average pump optical frequency, a condition that eliminates idler broadening. For digital phase modulation, the two pumps are phase modulated by separate phase modulators driven synchronously by a PRBS sequence and its complementary sequence, PRBS bar, respectively, or by another suitable sequence together with its complementary sequence.
A fiber parametric amplifier is an example of an application requiring high cw power in which a high-power pump amplifies an input signal(s) and creates one or more idlers at new wavelengths. However employing phase modulation to broaden the pump spectrum in order to suppress SBS has the undesirable consequence that the idlers experience broadening. This can degrade the idlers in various transmission and filtering environments. In U.S. Pat. No. 5,386,314 a technique was disclosed to prevent idler broadening in fiber parametric amplifiers. This patent disclosed that the distortion effects resulting from pump modulation used to raise the SBS threshold can be eliminated by using the same modulation, but opposite in sign, on the two pump signals. As a result, the phase conjugate mixing product of the input signal and the two modulated pump beams will therefore not have the low frequency distortion which normally accompanies SBS suppression by pump modulation.
More recently, the article by S. Radic, C. J. McKinstrie, R. M. Jopson, J. C. Centanni, A. R. Chraplyvy, C. G. Jorgensen, K. Brar and C. Headley, “Selective Suppression of Idler Spectral Broadening in Two-Pump Parametric Architectures”, IEEE Photon. Technol. Lett, V. 15, p673, May 1, 2003, discloses that while co-phased pump modulation allows the use of a single phase modulator, pump counter-phasing inherently requires two synchronized phase modulators. However, the electrooptic responses of two nominally identical high-speed phase modulators driven by nominally identical electronic amplifiers are never exactly the same. These differences in electronic and electrooptic responses can lead to degradation of the idler(s) because the pumps are not perfectly counter-phased.
Thus, there is a continuing need to reduce or eliminate SBS in fiber parametric applications to eliminate SBS without degrading idler(s).